1. Field of the Invention
The present invention generally relates to controlling an ignition coil. More specifically, the invention relates to a turn-on circuit for controlling an ignition coil.
2. Description of Related Art
In the area of ignition coils a high voltage zener diode is used in the standard design of a secondary circuit for a coil-per-plug (CPP) automotive ignition coil. The high voltage zener diode attenuates a voltage created in the secondary coil at the instant the coil is first turned on, also known as turn-on voltage or feed forward voltage. The high voltage zener diode precludes the feed forward voltage from causing early ignition.
The high voltage zener diode is a high cost component due to the high voltage value of the diode and its specialized purpose. The cost of the high voltage zener diode is a significant factor in the cost of the coil driver circuit and would represent a significant savings if eliminated. However, the high voltage zener diode in the prior art designs performs an essential function in reducing the feed forward voltage. Reducing the feed forward voltage prevents an over advanced spark which may cause early ignition and minimizes degradation of the spark gap. An over advanced spark could cause engine roughness, higher emissions, and increased fuel consumption.
In addition, removal of the high voltage zener diode may become vital for ODBII compliance, which mandates misfire detection. Ionization misfire detection with the ignition system is not possible if the high voltage zener diode is used because high voltage zener diode will block the ionization signal needed for misfire detection.
In view of the above, it is apparent that there exists a need for an improved circuit for controlling an ignition coil.